This invention relates to a process to effect impurity removal and upgrading of precious metals concentrates and residues so rendering them more amenable to downstream refining.
As is known in the art, subsequent to mining of precious metal ores and before final separation and purification of the precious metals in the refining process, precious metal residues and concentrates are produced and used as feedstock in the refining process. However, in addition to the precious metals, the residues and concentrates also include base metals (such as cobalt, copper, iron and nickel, for example), amphoterics (such as sulfur, selenium and tellurium, for example), and other impurities (including, but not limited to lead, zinc, tin, silver, arsenic, antimony and bismuth). The presence of these species significantly complicates the refining process.
A number of processes involving chlorination of precious metals feedstocks (concentrates produced by ore dressing, precious metals-bearing residues, precious metals-bearing matte and metal concentrates produced post leaching of base metals) have previously been disclosed in an attempt to effect upfront removal of base metals, amphoterics and other impurities.
CA 2,314,581 discloses a process comprising three (or four) steps of:                (i) subjecting the feedstock to a high temperature treatment with hydrogen chloride gas (HCl (g));        (ii) optional chlorination (under a chlorine atmosphere);        (iii) oxidation with oxygen; and        (iv) reduction (under a hydrogen atmosphere).        
Thermodynamic calculations have indicated that a disadvantage of this process is that the reaction of base metals, amphoterics and other impurities directly with hydrogen chloride gas requires a significant stoichiometric excess of hydrogen chloride. The reason for this is that for base metals the concentration of hydrogen chloride must be much higher than that of the hydrogen that is generated by the hydrochlorination reaction to permit the hydrochlorination reaction to proceed. Moreover, it has been observed that direct reaction of certain feedstocks with hydrogen chloride at elevated temperatures has the potential to result in the formation of an intermediate molten phase, which further complicates processing, in particular by limiting effective gas-solid contact. A further disadvantage to this process is that the secondary direct chlorination is prone to the formation of volatile precious metals chlorides, resulting in increased loss of precious metals.
CA 2,181,369 discloses a process including the steps of:                (i) exposing the feedstock to an oxidising environment (preferably oxygen or an oxygen containing gas mixture);        (ii) reduction (under hydrogen); and        (iii) chlorination using chlorine.        
The disclosed three step process requires a hydrogen reduction step. The act of reduction following oxidation not only adds an extra processing step, but further reduces metal oxide species back to metal prior to chlorination. In addition, the proposed step of final chlorination with chlorine or chlorine-containing gas results in a partial loss of precious metals.
U.S. Pat. No. 3,432,255 discloses a process for the recovery of molybdenum, tin, antimony, bismuth and like metal values from their ores and minerals. The source material is contacted with hydrogen chloride and an oxidizing gas at a temperature sufficiently high that the metal values are converted to volatile chloride compounds and removed from the reaction zone. The impurity metal values are said to remain as a residue of oxides which are substantially non-volatile under the process temperatures.
GB 1 502 765 and related applications U.S. Pat. No. 4,086,084 and U.S. Pat. No. 4,260,139 disclose a process for refining a metal concentrate comprising at least one precious metal, silver and a base metal, which base metal and silver form volatile halides so as to separate them from the precious metal(s). The process comprises contacting the metal concentrate in the form of a solid mixture with a halogen-containing gas at a temperature which is sufficiently high for the base metal and silver to form their halides and for said halides to volatilize from the solid mixture. The volatilized halides are removed from the solid precious metal-containing residue.
U.S. Pat. No. 4,092,152 discloses a method of reducing the presence of impurities from a sulphur-containing feed material such as dust and hydrometallurgical wastes that result from smelting and refining operations. Pelletized feed materials are heated to 800° C. to 1150° C. and reacted with a gas stream containing chlorine and oxygen. A limitation of the proposed treatment process is that it effects removal only of certain impurity species from precious metals, while failing to remove base metals and promoting precious metal losses.
Upfront removal of amphoterics, base metals and other impurities is expected to give rise to improved separation efficiencies, allowing for changes in and optimization of downstream refining processes and creation of additional refining capacity. It is accordingly an object of the present invention to provide a simple, effective and robust process of upgrading precious metal concentrates that will at least partially alleviate the disadvantages associated with existing technologies.